An explosion involving a tanker truck delivering liquid oxygen Wednesday afternoon outside a Kaiser Permanente medical office building prompted the evacuation of patients and employees at the campus north of Santa Rosa. Nearby Highway 101 was shut for about two hours.

Both medical offices located on the Old Redwood Highway property about a mile north of Kaiser’s main hospital campus were evacuated, with patients being escorted or taken away on stretchers. No patients or staff were hurt, but the tanker’s driver was transported to a nearby hospital for treatment of significant injuries.

One of the facilities, Building 4 at 3925 Old Redwood Highway, will open for business as usual Thursday, Kaiser said Wednesday evening. The other facility, Building 5 at 3975 Old Redwood Highway, will remain closed “for further inspections,” according to a statement from Kaiser.

The health care giant said staff were contacting patients who had appointments or outpatient surgeries set for Thursday at Medical Office Building 5 to reschedule or make other arrangements.

The explosion was reported around 2:30 p.m., and apparently occurred after something went wrong while the oxygen was being delivered to an on-site tank, according to Santa Rosa Assistant Fire Marshal Paul Lowenthal.

Jenna Ausiello, a field interviewer for the division of research at Kaiser’s regional office in Oakland, was interviewing someone in a conference room when the explosion happened.

“It sounded like the start of an earthquake,” she said. “That’s what I thought was happening. The windows shook ... it was just a big boom.”

Some witnesses reported the explosion could be heard in the nearby Fountaingrove area.

A small vegetation fire ignited, but it was quickly controlled by one of the first responding engines, Lowenthal said. Early reports indicated the truck may have crashed into the building, but officials hadn’t found any evidence to confirm that. The building wasn’t damaged, although some cars were, as was some mechanical equipment, Lowenthal said.

Authorities had the leak at the tanker under control by about 4:35 p.m., according to Lowenthal, though a small amount of “residual” gas was still seeping out about 5:20 p.m.

“That is being monitored by us, but that very small amount presents no threat to the surrounding area,” Lowenthal said.

Numerous Santa Rosa police officers and firefighters responded, with assistance from the Sheriff’s Office helicopter Henry 1. Authorities also were aided by a Santa Rosa Police Department drone, which was “instrumental” in giving officials a bird’s-eye view of the scene as they worked to get the situation under control, Lowenthal said.

Authorities couldn’t say exactly where the driver was at the time of the explosion.

The truck bore the name of Matheson, a New Jersey-based industrial and medical gas supplier. While parked in the back of the office campus, the rear of the vehicle appeared burned, with some metal parts disfigured.

Santa Rosa fire investigators were still working to determine the cause of the explosion, with assistance from Santa Rosa police. Matheson representatives are working “hand in hand” with authorities to resolve the situation, Lowenthal said.

Company officials reached in Newark, California, did not immediately respond to a request for comment.

The state Division of Occupational Safety and Health, better known as Cal/OSHA, is also investigating, Lowenthal said. Patients and staff either left on their own or were reportedly taken by Kaiser or Santa Rosa CityBus to Kaiser’s main campus on Bicentennial Way.

One patient inside Building 5 said she was in a pre-op room on the second floor, discussing her medication history with a nurse when she heard a “loud boom” and a felt a sudden “jolt” to the building.

“I didn’t know what it was. It sounded like there had been an explosion in the parking lot,” said Amy Landolt, 35, of Santa Rosa.

Landolt said her boyfriend thought maybe the building’s elevator had suddenly failed and came crashing down.

“There was a lot of confusion. Nobody really knew what happened,” Landolt said. “About 2 minutes later an evacuation notice came over the intercom and my nurse came in and said we need to evacuate immediately.”

Landolt described the evacuation as “really orderly” and that most people were “more confused than upset or scared.”

She said patients and staff gathered in the front of the building. Within about 10 minutes, police and fire personnel arrived, she said.

Kaiser nurse Ann Hicks was next door in Medical Office Building 4 with a patient in the pediatrics clinic when she head and felt a loud boom.

“We were all very concerned about it,” Hicks said.

She didn’t see what the sound was or smell anything out of the norm, but she said about 20-30 minutes later security announced an evacuation and everyone left in an orderly fashion.

The 62-year-old-man Ausiello was interviewing described the sound as “like if you had a concrete tower and you knocked it down in a demolition project.”

About 15 minutes went by before someone burst into the conference room and told the pair they had to leave. When they got outside, they saw numerous patients on stretchers being evacuated from the offices. The man described the scene as one of “bewilderment.”

Authorities kept telling the two to get further away, and eventually the pair joined many others walking south down Old Redwood Highway toward Fountain Grove Parkway, where Ausiello hoped to catch a Lyft.

Highway 101 was closed down in both directions, snarling traffic through much of Santa Rosa for about two hours.

“We understand, given the time of day, that was a significant impact to this part of Sonoma County,” Lowenthal said. “The goal was always to safely and efficiently get that leak secured as quickly as possible.”

A Kaiser employee said patients who were in the outpatient surgery center in Medical Office Building 5 were evacuated. The center’s surgery staff walked south to Kaiser’s main campus, 2 miles away on Bicentennial Way, said the Kaiser employee, who spoke anonymously because the employee is not authorized to speak to press.